Electrophoretic display method and device

ABSTRACT

An electrophoretic display device includes a first substrate, first and second driving electrodes arranged on the first substrate, a second substrate arranged in an opposing relation to the first substrate and a third driving electrode arranged on the second substrate. A transparent dielectric liquid is filled between the first substrate and the second substrate, and a plurality of migratory particles are dispersed in the transparent dielectric liquid. A barrier is disposed on a surface of the third driving electrode arranged on the second substrate and is situated in an opposing relation to a boundary between the first driving electrode and the second driving electrode.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an electrophoretic displaymethod and device in which charged migratory particles are migrated fordisplay of an image.

[0003] 2. Description of the Related Art

[0004] Recently, with rapid development of information equipment, theamount of data included in various kinds of information has increasedmore and more, and output of the information has been made in variousforms. Generally, information is outputted in two primary ways, i.e.,display-screen representation using a CRT or a liquid crystal, andhard-copy representation on paper using a printer or the like. In thedisplay-screen representation, increasing needs exist for a displaydevice that has low power consumption and is thin. Above all, a liquidcrystal display has been actively developed and commercialized as adisplay device adaptable for such needs.

[0005] However, a current liquid crystal display has problems, which arenot yet overcome to a satisfactory level, in that characters displayedon a screen become hard to perceive depending on the angle of viewingthe screen and the presence of reflected light, and a burden is imposedon a viewer's visual organ due to, e.g., flickering and low luminance ofa light source. Also, the display-screen representation using a CRT canprovide the contrast and luminance at a satisfactory high level ascompared with the case of using a liquid crystal display, but itaccompanies flickering, etc. and hence also cannot be regarded as havinga sufficient display quality as compared with the hard-copyrepresentation described below. Additionally, the display-screenrepresentation using a CRT entails a large and heavy body, and istherefore very poor in portability.

[0006] Meanwhile, at the beginning of the electronization era, it wasthought that the hard-copy representation would no longer be requiredwith the progress of electronization of information. In practice,however, a great deal of information is still outputted in the form ofhard copies. The reasons are as follows. When information is displayedusing a display unit, there occurs not only the above-mentioned problemswith regard to display quality, but also another problem that aresolution achieved by the display-screen representation is generallyabout 120 dpi at maximum, which is fairly lower than that in the case ofprinting out information on paper (usually not lower than 300 dpi).Accordingly, the display-screen representation imposes a greater burdenon a viewer's visual organ than the hard-copy representation. As aresult, although information can be confirmed on a display screen, theinformation is often outputted in the form of hard copies. Another majorreason why the hard-copy representation is utilized in spite of acapability of displaying information on a display screen, is that,unlike the display-screen representation, hard copies of information canbe arranged side by side in large number without being restricted by adisplay size defining a display area, and they can be rearranged orchecked in order with no need of complicated device operations.Furthermore, the hard-copy representation requires no energy for holdinginformation in a represented state, and has superior portabilityenabling information to be read or checked in any place and at any timeunless the amount of information is extremely large.

[0007] Thus, the hard-copy representation has various merits over thedisplay-screen representation so long as moving images or frequentrewriting is not needed, but it is disadvantageous in consuming a greatdeal of paper. In recent years, therefore, a rewritable recording medium(i.e., a recording medium that enables an image to be displayed in manyrecording and erasing cycles with high viewability, but does not requireenergy for holding the image in a displayed state) has been activelydeveloped. Such a third rewritable display system taking over superiorcharacteristics of hard copies is herein called a paper-like display.

[0008] Requirements of the paper-like display are, for example, that itis rewritable, requires no or a sufficiently small amount of energy forholding an image in a displayed state (memory character), has superiorportability, and has a high display quality. At present, one example ofa display system, which can be regarded as the paper-like display, is areversible display medium employing an organic low-molecular andhigh-molecular resin matrix and being able to record or erase an imageby a thermal printer head (e.g., see Japanese Patent Laid-Open Nos.55-154198 and 57-82086). Such a matrix is employed in display portionsof some prepaid cards, but still has problems that the contrast is notso high and the number of times at which an image can be recorded anderased repeatedly is relatively small, i.e., on the order of 150 to 500.

[0009] As another display system capable of being utilized as thepaper-like display, there is known an electrophoretic display device(U.S. Pat. No. 3,612,758) invented by Harold D. Lees, et al. Also,Japanese Patent Laid-Open No. 9-185087 discloses an electrophoreticdisplay device. Such a display device comprises a disperse systemwherein charged migratory particles are dispersed in a dielectricliquid, and a pair of electrodes is arranged in an opposing relationwith the disperse system situated between the electrodes. By applying avoltage to the disperse system through the electrodes, charged migratoryparticles are attracted under electrostatic forces to the side of theelectrode having a polarity opposite to that of charges of the migratoryparticles themselves based on the electrophoresis of charged particles.Display of information is performed by coloring the migratory particlesand utilizing a difference between the color of the migratory particlesand the color of the dyed dielectric liquid. More specifically, when themigratory particles are attracted onto the surface of a first electrodethat is closer to the viewer and is light transparent, the color of themigratory particles is observed. On the contrary, when the migratoryparticles are attracted onto the surface of a second electrode that isfarther away from the viewer, the color of the dielectric liquid, whichis dyed so as to have different optical characteristics from those ofthe migratory particles, is observed.

[0010] In the above-described electrophoretic display device, however, adye and a coloring material in the form of ions, for example, must bemixed in the dielectric liquid, and the presence of such a coloringmaterial tends to act as an unstable factor in the electrophoreticoperation because of giving rise to a new transfer of charges. Thistendency may deteriorate the performance, useful life and stability ofthe display device.

[0011] To overcome the above problem, Japanese Patent Laid-Open Nos.49-5598 and 11-202804 propose a display device wherein a pair ofelectrodes, i.e., first and second driving electrodes, are arranged onthe same substrate and migratory particles are migrated horizontally asviewed from the viewer. By applying voltages to the first and seconddriving electrodes, the migratory particles in a transparent dielectricliquid are horizontally migrated parallel to the substrate surfacebetween the first and second driving electrodes based on theelectrophoresis of charged particles, whereby an image is displayed.

[0012] In such an electrophoretic display device of the horizontallymigrating type, the dielectric liquid is transparent and the first andsecond driving electrodes have different colors as viewed from theviewer side such that the color of one electrode coincides with thecolor of the migratory particles. Assuming, for example, that the colorof the first driving electrode is black, the color of the second drivingelectrode is white, and the color of the migratory particles is black.The second driving electrode is exposed to provide a white view when themigratory particles are distributed over the first driving electrode,and the black color of the migratory particles is viewed when themigratory particles are distributed over the second driving electrode.

[0013] A display device comprising a large number of pixels arranged ina matrix pattern is electrically addressed in two primary ways, i.e., anactive matrix mode and a passive matrix mode.

[0014] In the active matrix mode, a switching element such as a thinfilm transistor (TFT) is formed corresponding to each pixel, andvoltages applied to the pixels are controlled in an independent mannerfor each pixel. By using the active matrix mode, the electrophoreticdisplay device of the horizontally migrating type can be operated with ahigh display contrast. However, the active matrix mode has problems thatthe process cost is relatively high and it is difficult to form thinfilm transistors on a polymer substrate because of a high processtemperature required in formation of the thin film transistors. Theseproblems are particularly critical to manufacture of a paper-likedisplay that is intended to be low in cost and flexible. A process forforming thin film transistors with a polymer material, which isadaptable for printing, is proposed to overcome those problems, but itis still an unknown quantity in practical applicability.

[0015] In the passive matrix mode, since only X-Y electrode lines arerequired as components necessary for addressing, the cost is relativelylow and the electrodes lines can be easily formed on a polymersubstrate. When applying a write voltage to a selected pixel, a voltagecorresponding to the write voltage is applied to the X- and Y-electrodelines that cross each other at a point defining the selected pixel. Ingeneral, however, where an electrophoretic display device is operated bythe passive matrix mode, there occurs so-called crosstalk, i.e., aphenomenon that the write voltage is applied to not only the selectedpixel but also other pixels around it, whereby the display contrast isnoticeably deteriorated. This is a problem that takes place inevitablybecause the electrophoretic display device does not have a definitethreshold characteristic with respect to the write voltage.

[0016] To cope with the above-mentioned problem, it has been proposed torealize the passive matrix addressing in an electrophoretic displaydevice, which does not have a threshold in principle, by employing athree-electrode structure wherein a control electrode is provided inaddition to a pair of display electrodes. Most proposals regarding thethree-electrode structure are related to electrophoretic display devicesof the type using vertically arranged electrodes, as disclosed in, byway of example, Japanese Patent Laid-Open No. 54-085699 (correspondingto U.S. Pat. No. 4,203,106).

[0017] For a three-electrode structure in the electrophoretic displaydevice of the horizontally migrating type, only one proposal isdisclosed in Japanese Patent Publication No. (by PCT application)8-507154 (corresponding to U.S. Pat. No. 5,345,251). However, a dispersesolution used in Japanese Patent Publication No. (by PCT application)8-507154 seems to be not transparent, but colored. This related arttherefore differs in category from the electrophoretic display devicesof the horizontally migrating type, which are featured by using atransparent disperse solution, as disclosed in the above-cited JapanesePatent Laid-Open Nos. 49-5598 and 11-202804 and as intended by thepresent invention.

[0018] Japanese Patent Publication No. (by PCT application) 8-507154discloses two types of constructions (FIGS. 17A and 17B of the attacheddrawings). In the first construction (FIG. 17A), a control electrode 14is arranged as a third electrode on the side of a second substrate 2 inan electrophoretic display device of the horizontally migrating type. Inthe second construction (FIG. 17B), a control electrode 14 is arrangedas a third electrode between a first driving electrode 3 and a seconddriving electrode 4 both arranged on the side of a first substrate 1.

[0019] In any type of the first and second constructions, the firstdriving electrode 3 in the forked form as an assembly of a plurality ofline electrodes and the second driving electrode 4 in the forked form asan assembly of a plurality of line electrodes, which are laid betweenadjacent lines of the first driving electrode 3, are both arranged onthe first substrate 1 within an area of each pixel. The second drivingelectrode 4 is arranged on a step 15 formed by a thick chrome film.Accordingly, a level difference 22 of about 0.3 μm is formed at theboundary between the first driving electrode 3 and the second drivingelectrode 4. In the first construction, the control electrode 14 isformed on the underside of the second substrate 2 over the entiresurface of each pixel area, the second substrate 2 being arranged in anopposing relation to the first substrate 1 with a spacing of 25 μm to116 μm left between both the electrodes. In the second construction, thecontrol electrode 14 is arranged on the first substrate 1 betweenrespective lines of the first driving electrode 3 and the second drivingelectrode 4. In FIGS. 17A and 17B, for the sake of explanation, thefirst driving electrode 3 and the second driving electrode 4 are eachillustrated as being constituted by one line.

[0020] The write operation of the electrophoretic display devicedisclosed in Japanese Patent Publication No. (by PCT application)8-507154 will be described with reference to FIGS. 18 and 19. FIG. 18shows migratory particles in respective operational conditions, and FIG.19 shows applied pulses and a change of reflectance. The cellconstruction is the same as that shown in FIG. 17A (except for only onepixel being shown in FIG. 18).

[0021] Note that values of applied voltages mentioned in the followingdescription are ones obtained under conditions of an experiment actuallyconducted by the inventors, and the conditions of the experiment are notexactly coincident with those described in Japanese Patent PublicationNo. (by PCT application) 8-507154. Such a discrepancy in thoseconditions primarily depends on differences in physical properties suchas the polarity and amount of charges on migratory particles used.Hereunder, the values of applied voltages, which were obtained asresults of the experiment made on the migratory particles used by theinventors, are employed for easier comparison with the operation of thepresent invention described later.

[0022] Also, although it seems that a colored liquid is used as adielectric solution in Japanese Patent Publication No. (by PCTapplication) 8-507154, a transparent dielectric liquid is used in thefollowing description for easier comparison with the operation of thepresent invention described later. Furthermore, for a method ofdeveloping display contrast, the following description is made as usinga similar method to that employed in embodiments of the presentinvention wherein the color of the migratory particles is black, thecolor of the first driving electrode is black, and the color of thesecond driving electrode is white.

[0023] It is supposed that the migratory particles 6 are positivelycharged, the first driving electrode 3 serves as a common electrode, anda driving voltage Vd and a control voltage Vc are applied respectivelyto the first driving electrode 3 and the control electrode 14 with theground potential of the second driving electrode 4 being as a reference.

[0024] In FIG. 8, a time period Ta represents a state where a white viewis maintained. Also, arrows schematically indicate vectors of anelectric field produced in a cell. The migratory particles 6 collectedover the first driving electrode 3 are restrained from moving toward theside of the second driving electrode 4 due to the presence of the leveldifference 22 between the first driving electrode 3 and the seconddriving electrode 4. At the same time, the migratory particles 6 areheld down to be urged toward the first substrate side under the controlvoltage Vc=+250 V applied between the first driving electrode 3 and thecontrol electrode 14. During this time period Ta, therefore, themigratory particles 6 are stabilized in a condition as shown and a whiteview state with a reflectance R of about 70% is maintained. The drivingvoltage Vd=+5 V applied to the first driving electrode 3 in a state, inwhich a current view is maintained, serves to suppress a tendency of themigratory particles 6 near the level difference 22 to migrate toward theside of the first driving electrode 3 in the black view maintainedstate.

[0025] In a write period Tb, the driving voltage Vd=+50 V and thecontrol voltage Vc=+50 V are applied. Since the first driving electrode3 and the control electrode 14 are set to the same potential, themigratory particles 6 are released from being held down under thecontrol voltage, whereby all of the migratory particles 6 arehorizontally migrated toward the side of the second driving electrode 4along the driving electrode surfaces beyond the level difference 22. Asa result, the reflectance R abruptly decreases.

[0026] In a time period Tc representing a state in which a black view ismaintained, the migratory particles 6 are held down to be urged towardthe first substrate side as shown under the control voltage Vc=+250 V.Accordingly, a black view state with a reflectance R of about 5% ismaintained.

[0027] The passive matrix addressing method disclosed in Japanese PatentPublication No. (by PCT application) 8-507154 will be described belowwith reference to FIGS. 20 and 21. Let us assume an electrophoreticdisplay device of the horizontally migrating type has an (m×n) matrixwherein m columns of pixels are arrayed in the X-direction and n rows ofpixels are arrayed in the Y-direction. Corresponding to the arrayconfiguration of pixels, a number m of data-signal electrode linesconnected to the control electrodes 14 are arranged in the columndirection, and a number n of scan-signal electrode lines connected tothe first driving electrodes 3 are arranged in the row direction, withboth the lines crossing each other in an orthogonal relation. The seconddriving electrode 4 is fixedly maintained at the ground potential so asto serve as a common electrode.

[0028] First, Vd=−50 V is applied to all of the scan-signal electrodelines and Vc=0 V is applied to all of the data-signal electrode lines sothat all of the migratory particles 6 are collected over the firstdriving electrode 3 (FIG. 20A, total erasure). Then, the scan-signalelectrode lines are selected one by one in sequence from the top in theY-direction for writing. In a selection period (write period), Vd=+50 Vis applied to the scan-signal electrode lines, Vc=+50 V is applied tothose ones of the data-signal electrode lines corresponding to selectedpixels, and Vc=+250 V is applied to the other ones of the data-signalelectrode lines corresponding to non-selected pixels. For the selectedpixels, the migratory particles 6 are migrated to the side of the seconddriving electrode 4 beyond the level difference under the drivingvoltage Vd=+50 V applied between the first and second driving electrode3, 4, whereby writing is performed (FIG. 20B). For the non-selectedpixels, the driving voltage Vd=+50 V is also applied to the firstdriving electrode 3. In the first construction, however, the migratoryparticles 6 are held down to be urged onto the first driving electrode 3under the control voltage Vc=+250 V and are prevented from migrating (toperform writing) (FIG. 20C).

[0029] On the other hand, in a non-selection period, Vd=+5 V is appliedto the scan-signal electrode lines, and Vc=+50 V or +250 V is applied tothe data-signal electrode lines (FIGS. 21A to 21D). In any case, themigratory particles 6 are held down to be urged onto the surface of thefirst substrate as shown under the control voltage.

[0030] Thus, writing of information is performed by the passive matrixaddressing method in the electrophoretic display device of thehorizontally migrating type that does not have a thresholdcharacteristic.

[0031] However, the following problems are experienced with theelectrophoretic display device of the horizontally migrating typedisclosed in Japanese Patent Publication No. (by PCT application)8-507154.

[0032] The disclosed first construction has a limitation that the leveldifference 22 defined by the step 15 cannot be set to a large value. Ifthe level difference is too large, part of the charged migratoryparticles 6 could not move over the level difference and would remain onthe lower one of two surfaces defining the level difference when forcedto migrate in the selection period, thus resulting in a reduced displaycontrast (FIG. 22A). To avoid the migratory particles 6 from remainingon the lower surface, the height of the step 15 must be limited to avalue approximately equal to the diameter of the migratory particles 6.

[0033] Due to such a limitation imposed on the height of the step 15,the level difference cannot provide the effect of suppressing themigration of the migratory particles 6 at a sufficient level.Accordingly, when applying the control voltage Vc to hold down themigration of the migratory particles 6 for the non-selected pixel (FIG.20C) in a condition where the driving voltage Vd is applied in theselection period, part of the migratory particles 6 moves over the leveldifference because of the step 15 being low. This phenomenon gives riseto a serious problem that crosstalk occurs and the display contrastdeteriorates (FIG. 22B).

[0034] If the control voltage Vc is set to a sufficiently high value,the undesired migration of the migratory particles 6 can be prevented toa nearly satisfactory extent. However, this solution not only has thedisadvantage of increasing the applied voltage, but also brings aboutanother problem that charges injected into dielectric components of thedevice under a high voltage remain there even after release of the highvoltage, and the operational condition of the migratory particles 6becomes unstable due to an unintended electric field caused by theremaining charges.

[0035] The limitation imposed on the height of the step 15 raises stillanother problem as follows. Since the height of the step 15 is notsufficient, the area difference between the first driving electrode 3and the second driving electrode 4 cannot be set to a large value. Ifthe area difference is set to a large value, the migratory particles 6would flow over onto the electrode surface having a larger area evenwhen the migratory particles 6 are urged such that they are allcollected on the electrode surface having a smaller area (FIG. 22C).Consequently, the display contrast is restricted because it isdetermined by an area ratio between the first driving electrode 3 andthe second driving electrode 4.

[0036] Further, in the disclosed first construction (FIG. 17A), theeffect of suppressing the migration of the migratory particles 6,provided by the level difference, is restricted only in the directiontoward the higher surface side from the lower surface side, whereas themigration of the migratory particles 6 from the higher surface side tothe lower surface side is rather accelerated. The write direction istherefore limited to only one direction from a white to black view. Inother words, the addressing method for writing is restricted to thesteps of first collecting the migratory particles 6 for an overallscreen to the lower surface side for total reset, and then writinginformation by migrating the migratory particles 6 in one direction tothe higher surface side. It is hence impossible to performbi-directional writing, i.e., black-to-white and white-to-black writing,and to realize such an operation as selectively rewriting only part ofan image on the screen.

[0037] The disclosed second construction (FIG. 17B) operates in theselection period such that a high voltage is applied to the controlelectrode 14 for the non-selected pixel to prevent the migratoryparticles 6 from moving in both directions, and the voltage of thecontrol electrode 14 is set to 0 V for the selected pixel, allowing themigratory particles 6 to smoothly migrate in either direction. In thiscase, therefore, the step 15 is considered to not be an essentialcomponent.

[0038] In the disclosed second construction, however, the controlelectrode 14 is able to control the migration of the migratory particles6 only between the first and second driving electrodes, and is unable tocontrol the migration of the migratory particles 6 within each of thedriving electrode surfaces. Due to a control voltage applied to thecontrol electrode 14 in the non-selection period, therefore, themigratory particles 6 having been evenly dispersed over the drivingelectrode surface are repellently migrated in a direction away from thecontrol electrode 14 and are partially distributed within the drivingelectrode surface as shown in FIG. 23A or 23B. This invites a problem ofnoticeably reducing the display contrast.

SUMMARY OF THE INVENTION

[0039] With the view of overcoming the problems set forth above, it isan object of the present invention to provide a novel electrophoreticdisplay method and device, which have the following features.

[0040] According to one aspect of the present invention, in anelectrophoretic display method for use in an electrophoretic displaydevice comprising a first substrate, first and second driving electrodesarranged on the first substrate, a second substrate arranged in anopposing relation to the first substrate, a third driving electrodearranged on the second substrate, a transparent dielectric liquid filledbetween the first substrate and the second substrate, and a plurality ofmigratory particles dispersed in the transparent dielectric liquid. Themethod comprises, for display of information, a first step of migratingthe migratory particles between the first driving electrode and thesecond driving electrode; and a second step of migrating the migratoryparticles between the first driving electrode or the second drivingelectrode and the third driving electrode.

[0041] Preferably, the electrophoretic display method further comprisesa step of applying voltages to the first driving electrode, the seconddriving electrode and the third driving electrode to provide a timeperiod in which a relationship of potentials of the first drivingelectrode and the second driving electrode being higher than a potentialof the third driving electrode is satisfied for positively chargedmigratory particles, or a time period in which a relationship ofpotentials of the first driving electrode and the second drivingelectrode being lower than a potential of the third driving electrode issatisfied for negatively charged migratory particles. The migratoryparticles are attracted onto the third driving electrode arranged on thesecond substrate.

[0042] Preferably, the electrophoretic display method further comprisesa step of rewriting display through a first stage of moving themigratory particles, which are attracted onto the third drivingelectrode, away from the third driving electrode, a second stage ofmigrating the migratory particles between the first driving electrodeand the second driving electrode, and a third stage of attracting themigratory particles onto the third driving electrode.

[0043] Preferably, the electrophoretic display method further comprisesa step of applying voltages to the first driving electrode, the seconddriving electrode and the third driving electrode to provide a timeperiod in which a relationship of potentials of the first drivingelectrode and the second driving electrode being lower than a potentialof the third driving electrode is satisfied for positively chargedmigratory particles, or a time period in which a relationship ofpotentials of the first driving electrode and the second drivingelectrode being higher than a potential of the third driving electrodeis satisfied for negatively charged migratory particles. The migratoryparticles are moved away from the third driving electrode arranged onthe second substrate.

[0044] As an alternative, preferably, the electrophoretic display methodfurther comprises a step of rewriting display through a first stage ofmoving the migratory particles, which are attracted onto the thirddriving electrode, away from the third driving electrode, andsimultaneously migrating the migratory particles onto the first drivingelectrode or the second driving electrode, and a second stage ofattracting the migratory particles to the second substrate side.

[0045] According to another aspect of the present invention, anelectrophoretic display device comprises a first substrate; first andsecond driving electrodes arranged on the first substrate; a secondsubstrate arranged in an opposing relation to the first substrate; athird driving electrode arranged on the second substrate; and atransparent dielectric liquid filled between the first substrate and thesecond substrate. A plurality of migratory particles are dispersed inthe transparent dielectric liquid, and a barrier is disposed on asurface of the third driving electrode arranged on the second substrate,with the barrier being situated in an opposing relation to a boundarybetween the first driving electrode and the second driving electrode.

[0046] According to still another aspect of the present invention, anelectrophoretic display device comprises a first substrate; first andsecond driving electrodes arranged on the first substrate; a secondsubstrate arranged in an opposing relation to the first substrate; athird driving electrode arranged on the second substrate; and atransparent dielectric liquid filled between the first substrate and thesecond substrate. A plurality of migratory particles are dispersed inthe transparent dielectric liquid, and a charged film disposed on asurface of the third driving electrode is arranged on the secondsubstrate, with the charged film having surface charges which areconstantly electrified with a polarity opposite to that of the chargedmigratory particles.

[0047] Preferably, the electrophoretic display device further comprisesinsulating layers arranged to cover the first driving electrode, thesecond driving electrode, and the third driving electrode.

[0048] Preferably, at least one of the first driving electrode, thesecond driving electrode, the third driving electrode, the firstsubstrate, the second substrate, and the insulating layers is colored ina color having different optical characteristics from those of themigratory particles.

[0049] Preferably, the first substrate and the second substrate are eachformed of a polymer film.

[0050] Preferably, an average diameter of the migratory particles is inthe range of 0.1 μm to 10 μm.

[0051] Preferably, the distance between the first substrate and thesecond substrate is not larger than 500 μm.

[0052] Preferably, the distance between the first substrate and thesecond substrate is not larger than 100 μm.

[0053] Preferably, the distance between the first substrate and thesecond substrate is not smaller than the diameter of the migratoryparticles.

[0054] Preferably, the distance between the first substrate and thesecond substrate is not smaller than twice the diameter of the migratoryparticles.

[0055] Preferably, the distance between the first substrate and thesecond substrate is not smaller than five times the diameter of themigratory particles.

[0056] Preferably, the first substrate and the migratory particles areblack or deep black in color.

[0057] With the electrophoretic display method and device set forthabove, the voltage required to inhibit the migration of the migratoryparticles and hold them at a standstill can be reduced to a large extentin comparison with that required in a conventional electrophoreticdisplay device disclosed in the above-cited Japanese Patent PublicationNo. (by PCT application) 8-507154, for example, when operated with aconventional passive matrix addressing method.

[0058] The present invention proposes a novel passive matrix addressingmethod based on a transfer display technique in which a pseudo thresholdis provided by transferring a display pattern onto the third drivingelectrode arranged on the second substrate. In other words, the passivematrix addressing method of the present invention differs basically fromthe conventional passive matrix addressing method in which a controlelectrode is employed to apply a high control voltage for holding downthe migratory particles and inhibiting the migration of them, asdisclosed in the above-cited Japanese Patent Publication No. (by PCTapplication) 8-507154.

[0059] More specifically, in accordance with the novel method of thepresent invention, the passive matrix addressing method can be realizedby migrating the migratory particles between the first driving electrodeand the second driving electrode to form a display pattern, and thenattracting the migratory particles onto the third driving electrodearranged on the second substrate, whereby the display pattern istransferred onto the second substrate side.

[0060] The reason why the novel passive matrix addressing method can berealized is based on two phenomena. First, since the migratory particlesare drawn under a driving voltage applied to the third driving electrodeand are attracted onto the third driving electrode, the migratoryparticles become hard to horizontally migrate under electric fieldsproduced by voltages applied to the first and second driving electrodes.Secondly, on the side of the second substrate that is disposed in anopposing relation to the first and second driving electrodes with acertain distance left between them, the electric fields produced by thevoltages applied to the first and second driving electrodes areweakened. Therefore, even when the voltages applied to the first andsecond driving electrodes are changed, the migratory particles avoidbeing affected by resulting changes of the electric fields.

[0061] Because the above two phenomena act effectively on the migratoryparticles, a high voltage is not required to be applied to the thirddriving electrode. Further, in a state where the display pattern istransferred onto the third driving electrode, even when the voltagesapplied to the first and second driving electrodes are changed, themigratory particles maintain the previous condition and the displaypattern formed before the voltage changes remains the same.Consequently, the present invention has a very valuable advantage that,in spite of the migratory particles not having a definite thresholdcharacteristic with respect to the driving voltage, it is possible toinhibit the migration of the migratory particles and hold them at astandstill even under a relatively low voltage, whereby anelectrophoretic display device can be operated with a passive matrixaddressing method in a satisfactory manner.

[0062] Further objects, features and advantages of the present inventionwill become apparent from the following description of the preferredembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0063]FIG. 1 is a sectional view of one typical example of a displaydevice of the present invention;

[0064]FIGS. 2A and 2B are each a plan view of one typical example of thedisplay device of the present invention;

[0065]FIGS. 3A and 3B are each a sectional view of another typicalexample of the display device of the present invention;

[0066]FIGS. 4A, 4B and 4C are explanatory views showing part of anaddressing method and operational conditions in the display device ofthe present invention;

[0067]FIGS. 5A, 5B and 5C are explanatory views showing other parts ofthe addressing method and operational conditions in the display deviceshown in FIG. 4;

[0068]FIGS. 6A to 6H are explanatory views showing part of one passivematrix addressing method for the display device of the presentinvention;

[0069]FIGS. 7A to 7F are explanatory views showing other parts of theone passive matrix addressing method for the display device shown inFIG. 6;

[0070]FIGS. 8A to 8F are explanatory views showing part of anotherpassive matrix addressing method for the display device of the presentinvention;

[0071]FIGS. 9A to 9D are explanatory views showing other parts ofanother passive matrix addressing method for the display device shown inFIG. 8;

[0072]FIG. 10 is a sectional view of still another typical example ofthe display device of the present invention;

[0073]FIG. 11 is a sectional view of still another typical example ofthe display device of the present invention;

[0074]FIG. 12 is a sectional view of still another typical example ofthe display device of the present invention;

[0075]FIGS. 13A and 13B are each a sectional view of still anothertypical example of the display device of the present invention;

[0076]FIG. 14 is a plan view showing a configuration of a 3×3 matrixfabricated in Example 1 of the present invention;

[0077]FIGS. 15A and 15B show respectively a time chart and a displaypattern for matrix addressing performed in Example 1 of the presentinvention;

[0078]FIGS. 16A and 16B show respectively a time chart and a displaypattern for matrix addressing performed in Example 2 of the presentinvention;

[0079]FIGS. 17A and 17B are each a sectional view of a conventionaldisplay device;

[0080]FIG. 18 shows an addressing method and operational conditions inone conventional display device;

[0081]FIG. 19 is a chart showing the addressing method and operationalconditions in the one conventional display device;

[0082]FIGS. 20A, 20B and 20C are explanatory views showing part of apassive matrix addressing method in the one conventional display device;

[0083]FIGS. 21A to 21D are explanatory views showing other parts of thepassive matrix addressing method in the one conventional display device;

[0084]FIGS. 22A, 22B and 22C are schematic views for explaining problemswith the one conventional display device;

[0085]FIGS. 23A and 22B are schematic views for explaining a problemwith the other conventional display device;

[0086]FIG. 24 is a plan view showing a configuration of a 3×3 matrixfabricated as a Comparative Example; and

[0087]FIGS. 25A and 25B show respectively a time chart and a displaypattern for matrix addressing performed in the Comparative Example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0088] Preferred embodiments will be described below with reference tothe drawings.

[0089] (Basic Construction and Operation)

[0090]FIG. 1 is a sectional view showing one typical example of aconstruction of a display device of the present invention. For the sakeof explanation, FIG. 1 illustrates the construction comprising twopixels. A first substrate 1 and a second substrate 2 are arranged in anopposing relation with partitions 10 provided between both substrates. Afirst driving electrode 3 and a second driving electrode 4 are formed onan upper surface of the first substrate 1, whereas a third drivingelectrode 5 is formed on a lower surface of the second substrate 2. Atransparent dielectric liquid 7 is filled in the space defined by boththe substrates 1, 2 and the partitions 10, and charged migratoryparticles 6 are dispersed in the dielectric liquid 7.

[0091] Plan shapes of the first driving electrode 3 and the seconddriving electrode 4 are not limited to particular ones. One typicalexample is a striped electrode (shown in FIG. 2A). In addition, eachelectrode may have any suitable shape such as being rectangular (FIG.2B) or in the form of a circular or other closed loop.

[0092] Practical sizes of the construction shown in FIG. 1 arepreferably set, by way of example, such that for a pixel size of 100μm×100 μm, the average diameter of the migratory particles 6 is 1 μm,the spacing between the first and second substrates 1, 2 is 80 μm, andan area ratio with respect to the total pixel area is 15% for the firstdriving electrode 3 and 85% for the second driving electrode 4.

[0093] Cell components of the display device can be colored in anydesired combination. For example, the color of the migratory particles 6is black, the second substrate 2 is transparent, the color of the firstdriving electrode 3 is black, the color of the second driving electrode4 is white, and the third driving electrode 5 is transparent. Thiscombination provides monochrome display in which a white view and ablack view can be selectively switched over when the viewer looks at thecell from the second substrate side. Color display is also possible bycoloring and arranging the second driving electrodes 4 so as to providered, green and blue pixels, or by forming the second driving electrode 4to be transparent and coloring the first substrate 1 so as to providered, green and blue pixels, or forming the second driving electrode 4 tobe transparent and providing an insulating layer 11 colored so as toprovide red, green and blue pixels on the first substrate 1, as shown inFIG. 3A.

[0094] The viewer is not always required to look at the cell from thesecond substrate side. When the viewer looks at the cell from the firstsubstrate side, a white view and a black view can be selectivelyswitched over to provide monochrome display by employing a combinationthat the color of the migratory particles 6 is black, the firstsubstrate 1 is transparent, the color of the first driving electrode 3is black, the second driving electrode 4 is transparent, and the colorof the third driving electrode 5 is white. Also, in this case, colordisplay is possible by coloring and arranging the third drivingelectrodes 5 so as to provide red, green and blue pixels, or by formingthe third driving electrode 5 to be transparent and coloring the secondsubstrate 2 so as to provide red, green and blue pixels, or forming thesecond driving electrode 4 to be transparent and providing an insulatinglayer 11 colored so as to provide red, green and blue pixels on theunderside of the second substrate 2, as shown in FIG. 3B.

[0095] Significant features of the addressing method used in the presentinvention will now be described with reference to FIGS. 4 and 5. FIGS. 4and 5 show operational conditions of the migratory particles insuccessive steps of the addressing method. The following description ismade on the premise that the migratory particles 6 are positivelycharged, and the components are colored such that the migratoryparticles 6 are black, the first driving electrode 3 is black, thesecond driving electrode 4 is white, and the third driving electrode 5is transparent. Arrows in FIGS. 4 and 5 schematically indicate vectorsof an electric field produced in the cell. The cell construction is thesame as that shown in FIG. 1. Further, it is assumed that a drivingvoltage Vd1 is applied to the first driving electrode 3, a drivingvoltage Vd2 is applied to the second driving electrode 4, and a drivingvoltage Vd3 is applied to the third driving electrode 5. Additionally,it is to be noted that voltage values indicated in the followingdescription do not represent ones which must be always employed inpractice, and can be set as desired so long as the display method of thepresent invention is realized.

[0096] First, the driving voltages are applied to the first drivingelectrode 3 and the second driving electrode 4 so that the migratoryparticles 6 are horizontally migrated (FIG. 4A). More specifically, fora pixel A, the driving voltage Vd1=−30 V is applied to the first drivingelectrode 3 and the driving voltage Vd2=+30 V is applied to the seconddriving electrode 4, thereby providing a white view state. For a pixelB, the driving voltage Vd1=+30 V is applied to the first drivingelectrode 3 and the driving voltage Vd2=−30 V is applied to the seconddriving electrode 4, thereby providing a black view state. At this time,the driving voltage Vd3=+60 V is applied to the third driving electrode5.

[0097] Subsequently, the driving voltage Vd3=−60 V is applied to thethird driving electrode 5, causing the migratory particles 6 to beattracted onto the third driving electrode 5 by electrostatic forces. Atthis time, as shown in FIG. 4B, the migratory particles 6 in pixel A aretransferred onto an area of the second substrate surface positioned inan opposing relation to the first driving electrode 3, and the migratoryparticles 6 in pixel B are transferred onto an area of the secondsubstrate surface positioned in an opposing relation to the seconddriving electrode 4. Therefore, the pixel A is held in the white viewstate and the pixel B is held in the black view state.

[0098] In the above condition, the polarity of the driving voltageapplied to each of the first and second driving electrodes is reverted.More specifically, for the pixel A, the driving voltage Vd1=+30 V isapplied to the first driving electrode 3 and the driving voltage Vd2−−30V is applied to the second driving electrode 4. For the pixel B, thedriving voltage Vd1=−30 V is applied to the first driving electrode 3and the driving voltage Vd2=+30 V is applied to the second drivingelectrode 4. In spite of such a reversal of the polarity, the displaystate is not changed (FIG. 4c).

[0099] Electric fields produced by the voltage applied to the firstdriving electrode 3 and the voltage applied to the second drivingelectrode 4 are strong on the first substrate side and are graduallyweakened as a point comes closer to the second substrate 2 away from thefirst substrate 1. Thus, the migratory particles 6 are substantiallyperfectly prevented from migrating horizontally because of twophenomena; i.e., the migratory particles 6 are attracted onto the thirddriving electrode 5 under the driving voltage applied to the thirddriving electrode 5, and the electric field tending to horizontallymigrate the migratory particles 6 is weak on the second substrate side.As compared with the construction disclosed in Japanese PatentPublication No. (by PCT application) 8-507154, therefore, the voltagerequired to inhibit the migration of the migratory particles 6 and holdthem at a standstill can be reduced to a large extent. This feature isthe most important advantage of the present invention. In other words,the above feature is very effective in driving an electrophoreticdisplay device, which does not have a definite threshold characteristicwith respect to the driving voltage, by the passive matrix addressingmethod.

[0100] The present invention proposes two types of addressing methodsfor rewriting of a displayed image.

[0101] [Rewriting Method 1]

[0102] According to one display rewriting method, as shown in FIG. 5A,the driving voltages Vd1=Vd2=0 V are applied to the first and seconddriving electrodes 3, 4, and the driving voltage Vd3=+60 V is applied tothe third driving electrode 5, whereby the migratory particles 6 aremigrated away from the second substrate 2 toward the first substrateside. Then, desired display is performed pixel by pixel. Assuming thatthe pixel A should provide a black view, the driving voltage Vd1=+30 Vis applied to the first driving electrode 3 and the driving voltageVd2=−30 V is applied to the second driving electrode 4. Also, assumingthat the pixel B should provide a white view, the driving voltage Vd1=−30 V is applied to the first driving electrode 3 and the drivingvoltage Vd2=+30 V is applied to the second driving electrode 4. Themigratory particles 6 are thereby migrated as desired (FIG. 5B).Subsequently, Vd3=−60 V is applied to the third driving electrode 5again for transferring the migratory particles i.e., the displaypattern, to the second substrate side as shown in FIG. 4B.

[0103] [Rewriting Method 2]

[0104] The other display rewriting method will now be described.According to this method, at the same time as applying the drivingvoltage Vd3=+60 V to the third driving electrode 5 to migrate themigratory particles 6 away from the second substrate 2 toward the firstsubstrate side, the driving voltages are applied to the first drivingelectrode 3 and the second driving electrode 4 for writing.

[0105] More specifically, as shown in FIG. 5C, at the moment when thedriving voltage Vd3=+60 V is applied to the third driving electrode 5,the driving voltage Vd1=+30 V is applied to the first driving electrode3 and the driving voltage Vd2=−30 V is applied to the second drivingelectrode 4 for the pixel A that should provide a black view, while thedriving voltage Vd1=−30 V is applied to the first driving electrode 3and the driving voltage Vd2=+30 V is applied to the second drivingelectrode 4 for the pixel B that should provide a white view. Themigratory particles 6 are thereby migrated onto the first drivingelectrode 3 or the second driving electrode 4 to form a display pattern.Subsequently, Vd3=−60 V is applied to the third driving electrode 5again for transferring the migratory particles, i.e., the displaypattern, to the second substrate side as shown in FIG. 4B. This methodis advantageous in shortening a write time.

[0106] (Passive Matrix Addressing Methods)

[0107] Two types of passive matrix addressing methods used in thepresent invention will be described below with reference to FIGS. 6 to9. Arrows in FIGS. 6 and 9 schematically indicate vectors of an electricfield produced in the cell. Let assume an electrophoretic display deviceof the horizontally migrating type that has an (m×n) matrix wherein mcolumns of pixels are arrayed in the X-direction and n rows of pixelsare arrayed in the Y-direction. Corresponding to the array configurationof pixels, a number m of first data-signal electrode lines connected tothe first driving electrodes 3 and a number m of second data-signalelectrode lines connected to the second driving electrode 4 are arrangedin the column direction, and a number n of scan-signal electrode linesconnected to the third driving electrodes 5 are arranged in the rowdirection, the two kinds of lines crossing each other in an orthogonalrelation.

[0108] [Passive Matrix Addressing Method 1]

[0109] Writing is performed in accordance with the above-describedRewriting Method 1 by selecting the scan-signal electrode lines one byone in sequence from the top in the Y-direction. The operation carriedout for the selected scan-signal electrode line, which is also simplycalled the selected line, is first described. It is assumed that, in thedisplay condition prior to the start of writing, a pixel to be rewrittento provide a white view is providing a black view (FIG. 6A), a pixel tobe rewritten to provide a black view is providing a white view (FIG.6B), and these black and white views are reversed upon writing.

[0110] In the selected line, as shown in FIGS. 6C and 6D, the drivingvoltage Vd3=+60 V is applied to the scan-signal electrode line, and atthe same time the driving voltages Vd1=Vd2=0 V are applied to the firstand second data-signal electrode lines, whereby the migratory particles6 are migrated away from the third driving electrode 5 toward the firstsubstrate side. Then, for each of those pixels in the selected linewhich should provide a white view, Vd1=30 V is applied to the firstdata-signal electrode line and Vd2=+30 V is applied to the seconddata-signal electrode line (FIG. 6E). Also, for each of those pixels inthe selected line which should provide a black view, Vd1=+30 V isapplied to the first data-signal electrode line and Vd2=−30 V is appliedto the second data-signal electrode line (FIG. 6F). With suchapplication of the driving voltages, the migratory particles 6 arehorizontally migrated onto the first driving electrode 3 or the seconddriving electrode 4, thereby providing desired display.

[0111] After completion of the horizontal migration of the migratoryparticles 6, Vd3=−60 V is applied to the relevant scan-signal electrodeline, whereupon the migratory particles 6 are attracted onto the thirddriving electrode 5 so that the display pattern is transferred to thesecond substrate side (FIGS. 6G and 6H). The operation to be carried outfor the selected line is thus ended, and the similar write operation isthen repeated for a next line as the selected line.

[0112] In the non-selected line, as shown in FIGS. 7A to 7F, the drivingvoltage Vd3=−60 V is continuously applied to the scan-signal electrodeline. To the first and second data-signal electrode lines, there areapplied −30 V or +30 V that is used when providing a white or blackview, and then 0 V that is used when migrating the migratory particles 6toward the first substrate side. However, since the migratory particles6 are still attracted onto the second substrate side under the drivingvoltage applied to the third driving electrode 5, they will nothorizontally migrate in spite of the driving voltages applied to thefirst and second data-signal electrode lines being changed between +30V, 0 V and −30 V. As a result, the previous display condition can beheld with stability.

[0113] [Passive Matrix Addressing Method 2]

[0114] Writing is performed in accordance with the above-describedRewriting Method 2 by selecting the scan-signal electrode lines one byone in sequence from the top in the Y-direction. The operation carriedout for the selected scan-signal electrode line, which is also simplycalled the selected line, is first described. As with the abovedescription of Passive Matrix Addressing Method 1, it is assumed that,in the display condition prior to the start of writing, a pixel to berewritten to provide a white view is providing a black view (FIG. 8A), apixel to be rewritten to provide a black view is providing a white view(FIG. 8B), and these black and white views are reversed upon writing.

[0115] In the selected line, the driving voltage Vd3=+60 V is applied tothe scan-signal electrode line for migrating the migratory particles 6toward the first substrate side. At the same time as applying thedriving voltage Vd3=+60 V, the respective driving voltages required forproviding desired display are applied to the first and seconddata-signal electrode lines, whereby the migratory particles 6 aremigrated onto the first driving electrode 3 or the second drivingelectrode 4. More specifically, for each of those pixels in the selectedline which should provide a white view, Vd1=−30 V is applied to thefirst data-signal electrode line and Vd2=+30 V is applied to the seconddata-signal electrode line (FIG. 8C). Also, for each of those pixels inthe selected line which should provide a black view, Vd1=+30 V isapplied to the first data-signal electrode line and Vd2=−30 V is appliedto the second data-signal electrode line (FIG. 8D). After completion ofthe migration of the migratory particles 6, Vd3=−60 V is applied to therelevant scan-signal electrode line, whereupon the migratory particles 6are attracted onto the third driving electrode 5 so that the displaypattern is transferred to the second substrate side (FIGS. 8E and 8F).

[0116] In the non-selected line, as shown in FIGS. 9A to 9D, the drivingvoltage Vd3=−60 V is continuously applied to the scan-signal electrodeline. To the first and second data-signal electrode lines, there areapplied −30 V or +30 V that is used when providing a white or blackview, and then 0 V that is used when migrating the migratory particles 6toward the first substrate side. However, since the migratory particles6 are still attracted onto the second substrate side under the drivingvoltage applied to the third driving electrode 5, they will nothorizontally migrate in spite of the driving voltages applied to thefirst and second data-signal electrode lines being changed between +30 Vand −30 V. As a result, the previous display condition can be held withstability.

[0117] Thus, in the electrophoretic display device of the horizontallymigrating type according to the present invention, a high quality imagecan be displayed by the passive matrix addressing without causing anycrosstalk.

[0118] (Variations of Construction)

[0119] The addressing method as the feature of the present invention isnot limited in its applications to the display device having theconstruction shown in FIG. 1. Other constructions of the display device,to which the addressing method of the present invention is effectivelyapplicable, will be described below with reference to the drawings.

[0120]FIG. 10 shows a construction of the display device of the presentinvention wherein a barrier 12 is provided as an obstacle on the surfaceof the third driving electrode 5 arranged on the underside of the secondsubstrate 2. More specifically, the barrier 12 is provided in anopposing relation to the boundary between the first driving electrode 3and the second driving electrode 4, and has a height several to severaltens times the diameter of the migratory particles 6. Thus, the barrier12 is featured in having a function to substantially inhibit thehorizontal migration of the migratory particles 6. When the migratoryparticles 6 are attracted onto the third driving electrode 5 after thewriting, the migratory particles can be prevented from migratinghorizontally by the presence of the barrier 12 even if the drivingvoltage applied to the third driving electrode 5 is lowered to reduceattraction forces toward the second substrate side. Accordingly, thebarrier 12 serves as a very effective means for achieving high-contrastdisplay at a relatively low voltage.

[0121]FIG. 11 shows another construction of the display device of thepresent invention wherein a charged film 13 is disposed on the surfaceof the third driving electrode 5 arranged on the underside of the secondsubstrate 2, the charged film 13 having surface charges which areconstantly electrified with a polarity opposite to that of the chargedmigratory particles 6. The charged film 13 is preferably made of aferroelectric material or an electret material. In addition to theelectrostatic forces produced by the driving voltage and acting on themigratory particles 6 to attract them onto the third driving electrode5, electrostatic forces produced by the surface charges of the chargedfilm 13 also act to draw the migratory particles and prevent them frommigrating horizontally. Therefore, even when the driving voltage forproducing the electrostatic forces to attract the migratory particles 6toward the third driving electrode 5 is lowered, high-contrast displaycan be provided.

[0122] The third driving electrode 5 arranged on the underside of thesecond substrate 2 is not always required to cover the entire pixel. Ina construction shown in FIG. 12, for example, a third driving electrode5 having a cutout formed in its part opposing to the boundary betweenthe first driving electrode 3 and the second driving electrode 4 isdisposed on the underside of the second substrate 2 in the constructionof FIG. 1. The construction of FIG. 12 is advantageous in that thedisplay pattern is more surely transferred with the migration of themigratory particles 6 in the write operation in accordance with theaddressing method described above with reference to FIGS. 4 and 5.

[0123] In the above description, one pair of the first driving electrode3 and the second driving electrode 4 is arranged in each pixel for thesake of explanation. However, the number of electrodes disposed in eachpixel is not limited to a particular value in the present invention, andas a matter of course plural pairs of electrodes may also be disposed ineach pixel. FIGS. 13A and 13B each show a construction in which twopairs of electrodes are disposed in each pixel. FIG. 13A corresponds tothe construction of FIG. 1, and FIG. 13B corresponds to the constructionof FIG. 10.

[0124] (Materials and Manufacturing Methods of Components)

[0125] The method of manufacturing the display device of this embodimentwill be described below with reference to FIG. 1. First, the firstdriving electrodes 3 and the second driving electrodes 4 are formed andpatterned into predetermined shapes on the first substrate 1. Then, thethird driving electrodes 5 are likewise formed and patterned intopredetermined shapes on the second substrate 2. Each substrate may bemade of any of inorganic materials including polymer films such aspolyethylene terephthalate (PET) and polyether sulfone (PES), glass, andquartz. The driving electrode may be made of any material so long as itis capable of patterning. Materials of the transparent electrode may be,e.g., indium tin oxide (ITO).

[0126] The surface of the driving electrode may be colored by utilizingthe color of an electrode material itself or the color of an insulatinglayer material itself formed on the electrode material, or by forming alayer of a material having a desired color on the electrode, theinsulating layer or the substrate surface. As an alternative, theinsulating layer, for example, may be mixed with a coloring material.

[0127] Subsequently, an insulating layer 8 is formed on both the firstdriving electrode 3 and the second driving electrode 4, and aninsulating layer 9 is formed on the third driving electrode 5. Materialsof each insulating layer are preferably hard to produce pinholes in theform of a thin film and have a low dielectric constant. Such materialsinclude, for example, amorphous fluorocarbon resins, highly transparentpolyimides, and PET. A film thickness of the insulating layer ispreferably on the order of 100 nm to 1 μm.

[0128] Then, the partitions 10 are formed on the second substrate 2. Aheight of each partition 10 is preferably not larger than 500 μm so thatflexibility is ensured. If the distance between the first substrate 1and the second substrate 2 is large, the transfer time of the displaypattern is prolonged and the driving voltage must be increased. From thepractical point of view, therefore, the partition height is preferablynot larger than 100 μm. Also, taking into account the diameter of themigratory particles 6, the partition height is preferably not smallerthan the particle diameter. Further, taking into account the migrationof the migratory particles between the first or second driving electrodeand the third driving electrode for transfer display, the partitionheight is preferably not smaller than twice the particle diameter.Moreover, in order that the migratory particles are less affected byelectric fields produced by the voltages applied to the first drivingelectrode and the second driving electrode in the non-selected line, thepartition height is preferably not smaller than five times the particlediameter.

[0129] The partitions 10 are not particularly limited in arrangement,but they are preferably arranged to surround each pixel so that themigratory particles 6 will not migrate between the pixels. A polymerresin is used as a material of the partitions 10. The partitions 10 maybe formed in any suitable manner. For example, the partitions 10 areformed by a method of coating a photosensitive resin layer andpatterning the layer through the steps of exposure and wet development,or a method of bonding partitions prepared separately, or a printingmethod. Alternatively, a method of forming partitions on the surface ofthe light-transparent first substrate by molding is also usable.

[0130] Then, the transparent dielectric liquid 7 and the migratoryparticles 6 are filled in each pixel space surrounded by the partitions10. A colorless transparent liquid, such as silicone oil, toluene,xylene, high-purity petroleum, is used as the dielectric liquid 7. Themigratory particles 6 being black are made of a material that exhibitsgood charging characteristics in the dielectric liquid 7. Such amaterial is, e.g., a resin, such as polyethylene or polystyrene, mixedwith carbon, etc. In consideration of the height of the partitions 10,the diameter of the migratory particles 6 is usually in the range ofabout 0.1 μm to 10 μm.

[0131] Then, after forming an adhesive layer on a joint surface of thefirst substrate 1 to the second substrate 2, the first and secondsubstrates 1, 2 are aligned with each other and bonded together underheating. A display device is completed by connecting voltage applyingmeans to the bonded assembly.

[0132] The barriers 12 on the second substrate 2, shown in FIG. 10, canbe formed using a material and a method similar to those used forforming the partitions 10. More specifically, a polymer resin is used asa material of the barriers 12. Further, the barriers 12 are formed by amethod of coating a photosensitive resin layer and patterning the layerthrough the steps of exposure and wet development, or a method ofbonding barriers prepared separately, or a printing method.Alternatively, a method of forming barriers on the surface of thelight-transparent second substrate by molding is also usable.

[0133] The charged film 13 on the second substrate 2, shown in FIG. 11,can be made of any of ferroelectric materials and electret materials.

[0134] When ferroelectric materials are used, preferable examples of thematerials include inorganic compounds such as lead zirconate titanate(PZT), lead lanthanum-added zirconate titanate (PLZT) and bariumtitanate, and organic polymers such as polyvinylidene fluoride (PVDF)and a copolymer of vinylidene fluoride and trifluoroethylene(PVDF/PTrFE). In this case, the charged film 13 can be formed by, e.g.,the sol-gel process, the sputtering process or the CVD (Chemical VaporDeposition) process.

[0135] When electret materials are used, fluorocarbon resins such asTeflon (Teflon-FEP and Teflon-TFE) provide superior characteristics.Other preferable materials are, for example, polyethylene,polypropylene, polystyrene, polymethyl methacrylate, polyvinyl chloride,polyethylene terephthalate, and polyimide. In this case, the chargedfilm 13 can be formed by, e.g., the thermo-electret process, theelectro-electret process, the radio-electret process, the photo-electretprocess, or the mechano-electret process.

EXAMPLES

[0136] The present invention will be described in more detail inconnection with Examples.

Example 1

[0137] In this Example, a (3×3)-matrix display cell having the cellconstruction shown in FIG. 1 was fabricated and operated in accordancewith the above-described Passive Matrix Addressing Method 1 to implementthe passive matrix addressing based on bi-directional writing. Thebi-directional writing is difficult to realize with the constructiondisclosed in the above-cited Japanese Patent Publication No. (by PCTapplication) 8-507154, and is one feature specific to the presentinvention. With this feature, the display cell of this Example is ableto perform the bi-directional writing, i.e., changes from a white toblack view and writing from a black to white view.

[0138]FIG. 14 is a plan view of the (3×3)-matrix display cell thusfabricated. The size of one pixel was 1 mm×1 mm, and the area ratio ofthe first driving electrode to the second driving electrode was 20:80.

[0139] A method of manufacturing the cell will be briefly describedbelow with reference to FIGS. 1 and 14. First, an insulating layer 8made of an acrylic resin containing a white pigment, such as alumina,dispersed therein was formed on an overall surface of a first substrate1 formed of a PET film having a thickness of 200 μm. Then, an ITO filmwas formed as a second driving electrode 4 on the insulating layer 8 ata low temperature and patterned into a shape as shown through the stepsof photolithography and dry etching. Then, a deep-black titanium carbidefilm was formed as a first driving electrode 3 on the insulating layer 8and patterned in a similar manner. Then, another insulating layer 8 madeof an amorphous fluorocarbon resin was formed in a thickness of 200 nmon the overall surface.

[0140] Subsequently, an ITO film was formed as a third driving electrode5 on a second substrate 2 formed of a PET film at a low temperature andpatterned into a shape as shown. An insulating layer 9 made of anamorphous fluorocarbon resin was then formed in a thickness of 200 nm onthe overall surface. Partitions 10 were formed on the insulating layer9. The partitions 10 were formed in a height of 70 μm by coating aphotosensitive epoxy resin and patterning the coated resin through thesteps of exposure and wet development. A dielectric liquid 7 and blackcharged migratory particles 6 were filled in each space surrounded bythe formed partitions 10.

[0141] Silicone oil was used as the dielectric liquid 7. A mixture ofpolystyrene and carbon, having an average particle diameter of about 1μm, was used as the black charged migratory particles 6. The migratoryparticles 6 were positively charged in the silicone oil. Then, a patternof thermally fusing adhesive layer was formed on a joint surface of thefirst substrate 1 to the second substrate 2, and the first substrate 1was placed on the partitions 10 formed on the second substrate 2 whileproperly aligning both the substrates with each other. The first andsecond driving electrodes 3, 4 were then bonded together under heating.A display device was completed by connecting voltage applying circuits(not shown) to the bonded assembly.

[0142] The addressing method in this Example will be described below.

[0143] The first driving electrodes 3 were used as first data-signalelectrode lines (D11-D13), the second driving electrodes 4 were used assecond data-signal electrode lines (D21-D23), and the third drivingelectrodes 5 were used as scan-signal electrode lines (S1-S3).

[0144]FIG. 15A is a time chart of driving voltages applied to the firstand second data-signal electrode lines and the scan-signal electrodelines, and FIG. 15B shows a change of the display condition in each timeperiod. In FIGS. 15A and 15B, each time period (T1, T2 or T3) is set to90 msec. Further, a time period A represents a period in which themigratory particles are moved away from the third driving electrode, andis set to 30 msec. A time period B represents a period in which themigratory particles are horizontally migrated, and is set to 30 msec. Atime period C represents a period in which a display pattern istransferred onto the third driving electrode, and is set to 30 msec.

[0145] Since the bi-directional writing is possible, an initialoperation to perform total reset is not required in this Example. It isassumed in this Example that a pattern shown in a time period T0 isgiven as an initial display pattern, and all pixels are reversed indisplay condition, i.e., color in view, for each of the scan-signalelectrode lines (S1-S3). Note that writing of information was performedin this Example in accordance with the Passive Matrix Addressing Method1 described above with reference to FIGS. 6 and 7. The detailed behaviorof the migratory particles in the write operation is similar to that inthe explanation of the Passive Matrix Addressing Method 1 and thereforeis not described herein.

[0146] The addressing method will now be described in sequence followingthe time chart of FIG. 15A. In the time period T1, the driving voltageswere applied to the respective lines in three stages. First, in the timeperiod A, Vd3=+60 V was applied to the scan-signal electrode line S1,which is selected at that time (i.e., a selected line), and Vd3=60 V wasapplied to the scan-signal electrode lines S2, S3, which are notselected at that time (i.e., non-selected lines). Also, 0 V was appliedto all of the first data-signal electrode lines (D11-D13) and all of thesecond data-signal electrode lines (D21-D23).

[0147] In the next time period B, as white-view writing voltages,Vd1=−30 V was applied to the first data-signal electrode lines D11, D13corresponding to the pixels (1,1) and (1,3), and Vd2=+30 V was appliedto the second data-signal electrode lines D21, D23 corresponding tothem. Also, as black-view writing voltages, Vd1=+30 V was applied to thefirst data-signal electrode line D12 corresponding to the pixel (1,2),and Vd2=−30 V was applied to the second data-signal electrode line D22corresponding to the same. As a result, all pixels in the scan-signalelectrode line S1 as the selected line were rewritten and reversed indisplay condition.

[0148] Then, in the time period C, Vd3=−60 V was applied to thescan-signal electrode line S1 as the selected line for transferring therewritten display pattern onto the second substrate. During the timeperiod T1, each of the pixels in the scan-signal electrode lines S2, S3as the non-selected lines was maintained in the initial displaycondition.

[0149] Subsequently, the addressing was successively performed in thetime periods T2 and T3 in a similar manner according to a selected pixelpattern. As a result, an objective reversed display pattern was obtainedwith a high contrast. A deterioration of contrast due to crosstalk andfailures in the migration and holding of the migratory particles was notobserved in the obtained display. An average contrast ratio of whiteview to black view was as high as about 10:1.

Comparative Example 1

[0150] As Comparative Example 1, a (3×3)-matrix display cell having thecell construction, shown in FIG. 17A, as disclosed in the above-citedJapanese Patent Publication No. (by PCT application) 8-507154 wasfabricated and operated in accordance with the passive matrix addressingbased on unidirectional writing.

[0151]FIG. 24 is a plan view of the (3×3)-matrix display cell thusfabricated. The size of one pixel was 1 mm×1 mm, and the area ratio ofthe first driving electrode 3 to the second driving electrode 4 was35:65. The spacing between the first substrate and the second substratewas 70 μm and the height of the step 15 was 0.3 μm. Positively chargedmigratory particles having an average particle diameter of 1 μm wereused. The driving electrodes and the migratory particles were colored inthe same manner as made in the construction of FIG. 1.

[0152] A method of manufacturing the cell will be briefly describedbelow with reference to FIGS. 17A and 24. First, an insulating layer 8made of an acrylic resin containing a white pigment, such as alumina,dispersed therein was formed on an overall surface of a first substrate1 formed of a PET film having a thickness of 200 μm. Then, a deep-blacktitanium carbide film was formed as a first driving electrode 3 on theinsulating layer 8 and patterned into a shape as shown through the stepsof photolithography and dry etching.

[0153] Then, an epoxy resin film was coated in a thickness of 0.3 μm,and in succession an ITO thin film was formed as a second drivingelectrode 4 at a low temperature by magnetron sputtering. Subsequently,a resist film was coated and patterned into a shape as shown. Finally,the first substrate 1 was subjected to reactive dry etching using CF₄and O₂ gases. As a result, a structural member having the second drivingelectrodes 4 arranged on the steps 15 with the height of 0.3 μm wasfabricated. Thereafter, another insulating layer 8 made of an amorphousfluorocarbon resin was formed in a thickness of 200 nm on the overallsurface.

[0154] Subsequently, an ITO film was formed as a control drivingelectrode 14 on a second substrate 2 formed of a PET film at a lowtemperature and patterned into a shape as shown. An insulating layer 9made of an amorphous fluorocarbon resin was then formed in a thicknessof 200 nm on the overall surface. Partitions 10 were formed on theinsulating layer 9. The partitions 10 were formed in a height of 70 μmby coating a photosensitive epoxy resin and patterning the coated resinthrough the steps of exposure and wet development. A dielectric liquid 7and black charged migratory particles 6 were filled in each spacesurrounded by the formed partitions 10.

[0155] The subsequent process is exactly the same as those describedabove in Example 1, and hence a description thereof is omitted herein.

[0156] The addressing method in Comparative Example 1 will be describedbelow.

[0157] The first driving electrodes 3 were used as scan-signal electrodelines (S1-S3), and the control electrodes 14 were used as data-signalelectrode lines (D11-D13). The second driving electrodes 4 were used ascommon electrodes and fixedly maintained at the ground potential.

[0158]FIG. 25A is a time chart of driving pulses applied to thescan-signal electrode lines and the data-signal electrode lines, andFIG. 25B shows a change of the display condition in each time period. InFIGS. 25A and 25B, each time period is set to 50 msec.

[0159] The addressing operation of the cell was started by initiallyresetting an overall screen to a white view. Then, in each of thescan-signal electrode lines, writing was performed in one direction(i.e., from a white to a black view) for selected pixels (1,2), (2,1)(2,3) and (3,2) corresponding to a set display pattern. Note thatwriting of information was performed in this Comparative Example 1 inaccordance with the addressing method described above with reference toFIGS. 18 to 21. The detailed behavior of the migratory particles in thewrite operation is similar to that in the explanation of the addressingmethod described above with reference to FIGS. 18 to 21, and thereforeis not described herein.

[0160] The addressing method will now be described in sequence followingthe time chart of FIG. 25A. In a time period TR, Vd=−50 V was applied toall of the scan-signal electrode lines S1 to S3, and Vc=0 V was appliedto all of the data-signal electrode lines D1-D3, thereby resetting allthe pixels to provide a white view.

[0161] Then, in a time period T1, Vd=+50 V was applied to thescan-signal electrode line S1, which is selected at that time (i.e., aselected line), and Vd=+5 V was applied to the scan-signal electrodelines S2, S3, which are not selected at that time (i.e., non-selectedlines). At the same time, the control voltage Vc=+50 V was applied tothe data-signal electrode line D2 corresponding to the selected pixel(1,2), and Vc=+250 V was applied to the data-signal electrode lines D1,D3 corresponding to the non-selected pixels (1,1), (1,3). As a result,only the selected pixel (1,2) in the selected scan-signal electrode lineS1 was rewritten to provide a black view, while a white view wasmaintained in the non-selected pixels (1,1), (1,3) in the selectedscan-signal electrode line S1 and each of the pixels in the non-selectedscan-signal electrode lines S2, S3. In the non-selected pixels (1,1) and(1,3), however, the migratory particles were not sufficiently held downeven under Vc=+250 V and part of the migratory particles was migrated tothe side of the second driving electrode as shown in FIG. 22C. Hence,the non-selected pixels (1,1) and (1,3) presented not a white view, buta gray view as shown in FIG. 25B.

[0162] Subsequently, the addressing was successively performed in timeperiods T2 and T3 in a similar manner according to a selected pixelpattern. As a result, an objective display pattern was obtained, but awhite view was entirely grayish and the display contrast was poor. Anaverage contrast ratio of white view to black view was about 3:1.

Example 2

[0163] In this Example 2, the (3×3)-matrix display cell employed inabove Example 1 was operated in accordance with the above-describedPassive Matrix Addressing Method 2 to implement the passive matrixaddressing based on bi-directional writing.

[0164] A display cell used in this Example has exactly the sameconstruction as that used in above Example 1 (plan view being shown inFIG. 14), and therefore an explanation of the manufacturing process isomitted herein.

[0165] The addressing method in this Example will be described below.

[0166] As with above Example 1, the first driving electrodes 3 were usedas first data-signal electrode lines (D11-D13), the second drivingelectrodes 4 were used as second data-signal electrode lines (D21-D23),and the third driving electrodes 5 were used as scan-signal electrodelines (S1-S3).

[0167]FIG. 16A is a time chart of driving voltages applied to the firstand second data-signal electrode lines and the scan-signal electrodelines, and FIG. 16B shows a change of the display condition in each timeperiod. In FIGS. 16A and 16B, each time period (T1, T2 or T3) is set to60 msec. Further, a time period A represents a period in which themigratory particles are moved away from the third driving electrode andmigrated onto the first or second driving electrode, and is set to 30msec. A time period B represents a period in which a display pattern istransferred onto the third driving electrode, and is set to 30 msec.

[0168] As with above Example 1, since the bi-directional writing ispossible, an initial operation to perform total reset is not required inthis Example. Also, it is assumed that a pattern shown in a time periodT0 is given as an initial display pattern, and all pixels are reversedin display condition, i.e., color in view, for each of the scan-signalelectrode lines (S1-S3). Note that writing of information was performedin this Example in accordance with the Passive Matrix Addressing Method2 described above with reference to FIGS. 8 and 9. The detailed behaviorof the migratory particles in the write operation is similar to that inthe explanation of the Passive Matrix Addressing Method 2 and thereforeis not described herein.

[0169] The addressing method will now be described in sequence followingthe time chart of FIG. 16A. In the time period T1, the driving voltageswere applied to the respective lines in two stages. In the first-halftime period A, Vd3=+60 V was applied to the selected scan-signalelectrode line S1, and Vd3=−60 V was applied to the non-selectedscan-signal electrode lines S2, S3. At the same time, as white-viewwriting voltages, Vd1=−30 V was applied to the first data-signalelectrode lines D11, D13 corresponding to the pixels (1,1) and (1,3),and Vd2=+30 V was applied to the second data-signal electrode lines D21,D23 corresponding to them. Also, as black-view writing voltages, Vd1=+30V was applied to the first data-signal electrode line D12 correspondingto the pixel (1,2), and Vd2=−30 V was applied to the second data-signalelectrode line D22 corresponding to the same. As a result, all pixels inthe selected scan-signal electrode line S1 were rewritten and reversedin display condition. Then, in the latter-half time period B, Vd3=−60 Vwas applied to the selected scan-signal electrode line S1 fortransferring the rewritten display pattern onto the third drivingelectrode 5. During the time period T1, each of the pixels in thenon-selected scan-signal electrode lines S2, S3 was maintained in theinitial display condition.

[0170] Subsequently, the addressing was successively performed in thetime periods T2 and T3 in a similar manner according to a selected pixelpattern. As a result, an objective reversed display pattern was obtainedwith a high contrast, and the objective pattern was displayed in ashorter time than required in above Example 1. A deterioration ofcontrast due to crosstalk and failures in the migration and holding ofthe migratory particles was not observed in the obtained display. Anaverage contrast ratio of white view to black view was as high as about10:1.

Example 3

[0171] In this Example 3, a (3×3)-matrix display cell having the cellconstruction shown in FIG. 10, wherein the barriers 12 were provided onthe surfaces of the third driving electrodes 5 arranged on the undersideof the second substrate 2, was fabricated and operated with the passivematrix addressing based on bi-directional writing.

[0172] A plan view of the (3×3)-matrix display cell thus fabricated wasthe same as that shown in FIG. 14. As with above Example 1, the size ofone pixel was 1 mm×1 mm, and the area ratio of the first drivingelectrode 3 to the second driving electrode 4 was 20:80.

[0173] A method of manufacturing the cell will be briefly describedbelow with reference to FIGS. 10 and 14.

[0174] First, an insulating layer 8 made of an acrylic resin containinga white pigment, such as alumina, dispersed therein was formed on anoverall surface of a first substrate 1 formed of a PET film having athickness of 200 μm. Then, an ITO film was formed as a second drivingelectrode 4 on the insulating layer 8 at a low temperature and patternedinto a shape as shown through the steps of photolithography and dryetching. Then, a deep-black titanium carbide film was formed as a firstdriving electrode 3 on the insulating layer 8 and patterned in a similarmanner. Then, another insulating layer 8 made of an amorphousfluorocarbon resin was formed in a thickness of 200 nm on the overallsurface. Partitions 10 were formed on this insulating layer 8. Thepartitions 10 were formed in a height of 70 μm by coating aphotosensitive epoxy resin and patterning the coated resin through thesteps of exposure and wet development. A dielectric liquid 7 and blackcharged migratory particles 6 were filled in each space surrounded bythe formed partitions 10.

[0175] Subsequently, an ITO film was formed as a third driving electrode5 on a second substrate 2 formed of a PET film at a low temperature andpatterned into a shape as shown. An insulating layer 9 made of anamorphous fluorocarbon resin was then formed in a thickness of 200 nm onthe overall surface. Barriers 12 were then formed in a thickness of 30μm on the insulating layer 9 through the steps of coating, exposing anddeveloping a photosensitive epoxy resin.

[0176] The subsequent process is exactly the same as those describedabove in Example 1, and hence a description thereof is omitted herein.

[0177] Matrix addressing was performed exactly in the same manner asthat in Example 1. As a result, an objective display pattern wasobtained with a higher contrast because of the presence of the barriers12. Further, the driving voltage applied for transferring the displaypattern onto the third driving electrode could be reduced to Vd3=−45 V.A deterioration of contrast due to crosstalk and failures in themigration and holding of the migratory particles was not observed in theobtained display. An average contrast ratio of white view to black viewwas as high as about 15:1.

Example 4

[0178] In this Example 4, a (3×3)-matrix display cell having the cellconstruction shown in FIG. 11, wherein the charged film 13 was formed onthe surfaces of the third driving electrodes 5 arranged on the undersideof the second substrate 2, was fabricated and operated with the passivematrix addressing based on bi-directional writing.

[0179] A plan view of the (3×3)-matrix display cell thus fabricated wasthe same as that shown in FIG. 14. As with above Example 1, the size ofone pixel was 1 mm×1 mm, and the area ratio of the first drivingelectrode 3 to the second driving electrode 4 was 20:80.

[0180] A method of manufacturing the cell will be briefly describedbelow with reference to FIGS. 11 and 14.

[0181] First, an insulating layer 8 made of an acrylic resin containinga white pigment, such as alumina, dispersed therein was formed on anoverall surface of a first substrate 1 formed of a PET film having athickness of 200 μm. Then, an ITO film was formed as a second drivingelectrode 4 on the insulating layer 8 at a low temperature and patternedinto a shape as shown through the steps of photolithography and dryetching. Then, a deep-black titanium carbide film was formed as a firstdriving electrode 3 on the insulating layer 8 and patterned in a similarmanner. Then, another insulating layer 8 made of an amorphousfluorocarbon resin was formed in a thickness of 200 nm on the overallsurface. Partitions 10 were formed on this insulating layer 8. Thepartitions 10 were formed in a height of 70 μm by coating aphotosensitive epoxy resin and patterning the coated resin through thesteps of exposure and wet development. A dielectric liquid 7 and blackcharged migratory particles 6 were filled in each space surrounded bythe formed partitions 10.

[0182] Subsequently, an ITO film was formed as a third driving electrode5 on a second substrate 2 formed of a PET film at a low temperature andpatterned into a shape as shown. An insulating layer 9 made of anamorphous fluorocarbon resin was then formed in a thickness of 200 nm onthe overall surface.

[0183] The charged film 13 was then formed. Teflon-FEP was used as amaterial of the charged film 13, and was treated to have an electretproperty with a corona discharge under heating at a high temperature.More specifically, the insulating layer 9 was etched by Ar gas for fiveminutes, whereby the layer surface was roughed to increase adhesion ofthe charged film onto it. After laying a transparent Teflon-FEP sheethaving a thickness of 5 μm on the roughed surface, the sheet was heatedand fused at 300° C. while a weight was imposed on the sheet through aglass plate. Then, by cooling the sheet, a Teflon-FEP film was formed ina thickness of 5 μm on the third driving electrode 5. For treatment togive the Teflon-FEP sheet an electret property, a knife edge electrodeattached to an XYZ displacement mechanism and the second substrate,including the Teflon-FEP film and the electrode films formed thereon,were both placed in a thermostatic chamber. The knife edge electrode wasarranged to face a surface of the Teflon-FEP film through a gap, and thedistance (gap) between the knife edge electrode and the Teflon-FEP filmwas adjusted to 200 μm. While maintaining an inner space of thethermostatic chamber at 300° C., a voltage of 5 kV was applied betweenthe electrode films and the knife edge electrode in such a directionthat the knife edge electrode was on the negative side, therebygenerating a corona discharge between the electrode films and the knifeedge electrode. The knife edge electrode was moved to reciprocate at aconstant speed in a horizontal direction parallel to the substratesurface by the XYZ displacement mechanism supporting the knife edgeelectrode. The overall substrate surface was thereby subjected touniform irradiation of the corona discharge. The treatment to give theTeflon-FEP sheet an electret property was completed by rapidly coolingthe irradiated substrate surface by dry nitrogen. The charged film thusobtained was transparent and a measured surface potential of the chargedfilm was −20 V.

[0184] The subsequent process is exactly the same as those describedabove in Example 1, and hence a description thereof is omitted herein.

[0185] Matrix addressing was performed exactly in the same manner asthat in Example 1. As a result, an objective display pattern wasachieved in an addressing time comparable to that in Example 1. In otherwords, it was confirmed that the addressing characteristics were hardlyaffected by attraction of the migratory particles by the charged film13. Further, the objective display pattern was obtained with a highercontrast because of the presence of the charged film 13. In addition,the driving voltage applied for transferring the display pattern ontothe third driving electrode could be reduced to Vd3=−40 V. Adeterioration of contrast due to crosstalk and failures in the migrationand holding of the migratory particles was not observed in the obtaineddisplay. An average contrast ratio of white view to black view was ashigh as about 13:1.

Example 5

[0186] In this Example 5, a cell having the construction shown in FIG. 3was fabricated and operated to provide color display by forming red,green and blue pixels on the first substrate 1 in a combinedarrangement. The size of one pixel was 1 mm×1 mm, and the area ratio ofthe first driving electrode 3 to the second driving electrode 4 was20:80.

[0187] A method of manufacturing a red cell will be briefly describedbelow with reference to FIG. 3A.

[0188] First, a colored insulating layer 11 made of an acrylic resincontaining a red pigment dispersed therein was formed on a firstsubstrate 1 formed of a PET film having a thickness of 200 μm. Then, adeep-black titanium carbide film was formed as a first driving electrode3 on the insulating layer 11 and patterned into a shape as shown throughthe steps of photolithography and dry etching.

[0189] Then, an amorphous fluorocarbon resin was coated in a thicknessof 100 nm, and in succession an ITO thin film was formed as a seconddriving electrode 4 at a low temperature by magnetron sputtering.Subsequently, a resist film was coated and patterned into a shape asshown. Finally, the first substrate 1 was subjected to reactive dryetching using CF₄ and O₂ gases, whereby the second driving electrode 4of ITO was formed. Thereafter, an insulating layer 8 made of anamorphous fluorocarbon resin was formed in a thickness of 200 nm on theoverall surface.

[0190] The subsequent process is exactly the same as those describedabove in Example 1, and hence a description thereof is omitted herein.

[0191] The red cell thus fabricated was operated to provide display inaccordance with the method described above in connection with FIGS. 4and 5. First, the driving voltage Vd1=−30 V was applied to the firstdriving electrode 3 and the driving voltage Vd2=+30 V was applied to thesecond driving electrode 4 for migrating the migratory particles 6 so asto position on the side of the first driving electrode 3. Then, thedriving voltage Vd3=−60 V was applied to the third driving electrode 5,causing the migratory particles 6 to be attracted onto the third drivingelectrode 5 by electrostatic forces. At this time, when looking at thecell from the second substrate side, the cell presented a red viewbecause the red colored insulating layer 11 formed under the transparentsecond driving electrode 4 was observed by the viewer.

[0192] Subsequently, the display condition was rewritten in accordancewith the Passive Matrix Addressing Method 1. The driving voltagesVd1=Vd2=0 V were applied to the first and second driving electrodes 3, 4and the driving voltage Vd3=+60 V was applied to the third drivingelectrode 5, whereby the migratory particles 6 were migrated away fromthe second substrate 2 toward the first substrate side. Thereafter, thedriving voltage Vd1=+30 V was applied to the first driving electrode 3and the driving voltage Vd2=−30 V was applied to the second drivingelectrode 4 for migrating the migratory particles 6 so as to position onthe side of the second driving electrode 4. Then, the driving voltageVd3=−60 V was applied again to the third driving electrode 5, causingthe migratory particles 6 to be transferred onto the second substrateside. At this time, the cell presented a black view because the blackmigratory particles 6 and the deep-black first driving electrode 3 wereobserved from the second substrate side. The display condition could berewritten in a time of not longer than 50 msec.

[0193] Successively, the display condition was rewritten in accordancewith the Passive Matrix Addressing Method 2. The driving voltage Vd3=+60V was applied to the third driving electrode 5 for migrating themigratory particles 6 away from the second substrate 2 toward the firstsubstrate side. Simultaneously, the driving voltage Vd1=−30 V wasapplied to the first driving electrode 3 and the driving voltage Vd2=+30V was applied to the second driving electrode 4 for migrating themigratory particles 6 so as to position on the side of the first drivingelectrode 3. Then, the driving voltage Vd3=−60 V was applied to thethird driving electrode 5, causing the migratory particles 6 to betransferred onto the second substrate side. At this time, the cellpresented a red view because the red colored insulating layer 11 formedunder the transparent second driving electrode 4 was observed from thesecond substrate side. The display condition could be rewritten in atime of not longer than 30 msec, and the rewrite speed was increased incomparison with the case of using the Passive Matrix Addressing Method1.

[0194] A green cell was fabricated through the same process as for thered cell except for forming, on a first substrate 1 formed of a PETfilm, a colored insulating layer 11 made of an acrylic resin containinga green pigment dispersed therein. The green cell thus fabricated wasoperated to provide display in accordance with the same method as forthe red cell. As a result, the green cell was able to present a greenview as intended.

[0195] A blue cell was also fabricated through the same process as forthe red cell except for forming, on a first substrate 1 formed of a PETfilm, a colored insulating layer 11 made of an acrylic resin containinga blue pigment dispersed therein. The blue cell thus fabricated wasoperated to provide display in accordance with the same method as forthe red cell. As a result, the blue cell was able to present a blue viewas intended.

[0196] Three types of cells representing red, green and blue pixels werefabricated in a combined arrangement by forming the colored insulatinglayer 11 in three colors of red, green and blue. As a result, colordisplay was obtained by those cells. In other words, color display couldbe realized by fabricating three cells each having the construction ofFIG. 3A to provide red, green and blue pixels, and arranging those threepixels adjacent to each other so as to provide one composite pixel.

[0197] As described above in detail, the present invention can providethe following advantages.

[0198] First, in the selected line, the migratory particles forming adisplay pattern are attracted onto the third driving electrode, and arekept away from the first and second driving electrodes. Therefore, evenwhen the driving voltages applied to the first and second drivingelectrodes are changed, the migratory particles are less affected by thechange of an electric field, and the display pattern is avoided fromchanging due to the unintended migration of the migratory particles. Itis thus possible to surely inhibit the unintended migration of themigratory particles and hold them in a desired position under a lowervoltage.

[0199] Secondly, in an electrophoretic display device of thehorizontally migrating type, passive matrix addressing is realized witha high display contrast without causing any crosstalk. The reason isthat the novel addressing method has succeeded in substantiallyperfectly eliminating the occurrence of crosstalk, which has beenexperienced in the conventional device due to a failure in holding themigratory particles properly in the non-selected pixels.

[0200] Thirdly, bi-directional writing is enabled in the presentinvention. Therefore, initial total reset is no longer required, andpartial rewriting to rewrite only part of a display screen is realized.

[0201] Fourthly, by forming the barriers or the charged film, thedriving voltage can be further reduced which is required for inhibitingthe migration of the migratory particles and holding them at astandstill in a state where the display pattern is transferred onto thethird driving electrode. In addition, the contrast is improved.

[0202] Fifthly, color display can be realized.

[0203] While the present invention has been described with reference towhat are presently considered to be the preferred embodiments, it is tobe understood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

What is claimed is:
 1. An electrophoretic display method for use in anelectrophoretic display device comprising a first substrate, first andsecond driving electrodes arranged on the first substrate, a secondsubstrate arranged in an opposing relation to the first substrate, athird driving electrode arranged on the second substrate, a transparentdielectric liquid filled between the first substrate and the secondsubstrate, and a plurality of migratory particles dispersed in thetransparent dielectric liquid, the method comprising the steps of:migrating the migratory particles between the first driving electrodeand the second driving electrode; and migrating the migratory particlesbetween the first driving electrode or the second driving electrode andthe third driving electrode.
 2. An electrophoretic display methodaccording to claim 1 , further comprising the step of applying voltagesto the first driving electrode, the second driving electrode and thethird driving electrode to provide a time period in which a relationshipof potentials of the first driving electrode and the second drivingelectrode being higher than a potential of the third driving electrodeis satisfied for positively charged migratory particles, or a timeperiod in which a relationship of potentials of the first drivingelectrode and the second driving electrode being lower than a potentialof the third driving electrode is satisfied for negatively chargedmigratory particles, whereby the migratory particles are attracted ontothe third driving electrode arranged on the second substrate.
 3. Anelectrophoretic display method according to claim 1 , further comprisingthe step of rewriting a display through a first stage of moving themigratory particles, which are attracted to the third driving electrode,away from the third driving electrode, a second stage of migrating themigratory particles between the first driving electrode and the seconddriving electrode, and a third stage of attracting the migratoryparticles onto the third driving electrode.
 4. An electrophoreticdisplay method according to claim 1 , further comprising the step ofapplying voltages to the first driving electrode, the second drivingelectrode and the third driving electrode to provide a time period inwhich a relationship of potentials of the first driving electrode andthe second driving electrode being lower than a potential of the thirddriving electrode is satisfied for positively charged migratoryparticles, or a time period in which a relationship of potentials of thefirst driving electrode and the second driving electrode being higherthan a potential of the third driving electrode is satisfied fornegatively charged migratory particles, whereby the migratory particlesare moved away from the third driving electrode arranged on the secondsubstrate.
 5. An electrophoretic display method according to claim 1 ,further comprising the step of rewriting display through a first stageof moving the migratory particles, which are attracted onto the thirddriving electrode, away from the third driving electrode, andsimultaneously migrating the migratory particles onto the first drivingelectrode or the second driving electrode, and a second stage ofattracting the migratory particles to the second substrate side.
 6. Anelectrophoretic display device comprising: a first substrate; first andsecond driving electrodes arranged on said first substrate; a secondsubstrate arranged in an opposing relation to said first substrate; athird driving electrode arranged on said second substrate; a transparentdielectric liquid filled between said first substrate and said secondsubstrate; a plurality of migratory particles dispersed in saidtransparent dielectric liquid; and a barrier disposed on a surface ofsaid third driving electrode arranged on said second substrate, saidbarrier being situated in an opposing relation to a boundary betweensaid first driving electrode and said second driving electrode.
 7. Anelectrophoretic display device comprising: a first substrate; first andsecond driving electrodes arranged on said first substrate; a secondsubstrate arranged in an opposing relation to said first substrate; athird driving electrode arranged on said second substrate; a transparentdielectric liquid filled between said first substrate and said secondsubstrate; a plurality of migratory particles dispersed in saidtransparent dielectric liquid; and a charged film disposed on a surfaceof said third driving electrode arranged on said second substrate, saidcharged film having surface charges which are constantly electrifiedwith a polarity opposite to that of the charged migratory particles. 8.An electrophoretic display device according to claim 6 or 7 , furthercomprising insulating layers arranged to cover said first drivingelectrode, said second driving electrode, and said third drivingelectrode.
 9. An electrophoretic display device according to claim 6 or7 , wherein at least one of said first driving electrode, said seconddriving electrode, said third driving electrode, said first substrate,said second substrate, and said insulating layers is colored to havedifferent optical characteristics from those of the migratory particles.10. An electrophoretic display device according to claim 6 or 7 ,wherein said first substrate and said second substrate are each formedof a polymer film.
 11. An electrophoretic display device according toclaim 6 or 7 , wherein an average diameter of the migratory particles isin the range of 0.1 μm to 10 μm.
 12. An electrophoretic display deviceaccording to claim 6 or 7 , wherein the distance between said firstsubstrate and said second substrate is not larger than 500 μm.
 13. Anelectrophoretic display device according to claim 6 or 7 , wherein thedistance between said first substrate and said second substrate is notlarger than 100 μm.
 14. An electrophoretic display device according toclaim 6 or 7 , wherein the distance between said first substrate andsaid second substrate is not smaller than the diameter of the migratoryparticles.
 15. An electrophoretic display device according to claim 6 or7 , wherein the distance between said first substrate and said secondsubstrate is not smaller than twice the diameter of the migratoryparticles.
 16. An electrophoretic display device according to claim 6 or7 , wherein the distance between said first substrate and said secondsubstrate is not smaller than five times the diameter of the migratoryparticles.
 17. An electrophoretic display device according to claim 6 or7 , wherein said first substrate and the migratory particles are blackor deep black in color.